2.5 IR2 Conversion

The actual selection of VOPs for implementing call/return for a
given function is handled in ir2tran.lisp. Returns are handled
by ir2-convert-return, calls are handled by ir2-convert-local-call,
ir2-convert-full-call, and ir2-convert-mv-call, and
function prologues are handled by ir2-convert-bind (which calls
init-xep-environment for the case of an entry point for a full
call).